from
https://www.nature.com/articles/d41586-023-03230-z

ESSAY
16 October 2023
How would we know whether there is life on Earth? This bold experiment
found out
Thirty years ago, astronomer Carl Sagan convinced NASA to turn a passing
space probe’s instruments on Earth to look for life — with results that
still reverberate today.
Alexandra Witze
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First frame of the Galileo Earth spin movie, a 500- frame time-lapse
motion picture.
Anything down there? Earth as seen by the Galileo probe in 1990.Credit:
NASA/JPL

It began the way many discoveries do — a tickling of curiosity in the
back of someone’s mind. That someone was astronomer and communicator
Carl Sagan. The thing doing the tickling was the trajectory of NASA’s
Galileo spacecraft, which had launched in October 1989 and was the first
to orbit Jupiter. The result was a paper in Nature 30 years ago this
week that changed how scientists thought about looking for life on other
planets.

The opportunity stemmed from a tragic mishap. Almost four years before
Galileo’s launch, in January 1986, the space shuttle Challenger had
exploded shortly after lift-off, taking seven lives with it. NASA
cancelled its plans to dispatch Galileo on a speedy path to Jupiter
using a liquid-fuelled rocket aboard another space shuttle. Instead, the
probe was released more gently from an orbiting shuttle, with mission
engineers slingshotting it around Venus and Earth so it could gain the
gravitational boosts that would catapult it all the way to Jupiter.

On 8 December 1990, Galileo was due to skim past Earth, just 960
kilometres above the surface. The tickling became an itch that Sagan had
to scratch. He talked NASA into pointing the spacecraft’s instruments at
our planet. The resulting paper was titled ‘A search for life on Earth
from the Galileo spacecraft’1.

The outside view
We are in a unique position of knowing that life exists on Earth. To use
our own home to test whether we could discern that remotely was an
extraordinary suggestion at the time, when so little was known about the
environments in which life might thrive. “It’s almost like a
science-fiction story wrapped up in a paper,” says David Grinspoon,
senior scientist for astrobiology strategy at NASA’s headquarters in
Washington DC. “Let’s imagine that we’re seeing Earth for the first time.”

It came at a time, too, when the search for life elsewhere in the Solar
System was at a low ebb. US and Soviet robotic missions in the 1960s and
1970s had revealed that Venus — once thought to be a haven for exotic
organisms — was hellishly hot beneath its dense clouds of carbon
dioxide. Mars, crisscrossed by the ‘irrigation canals’ of astronomers’
imagination2, was a seemingly barren wasteland. In 1990, no one yet knew
about the buried oceans that lay on Jupiter’s moon Europa — a discovery
that Galileo would go on to make3 — or on Saturn’s moon Enceladus, both
of which are now seen as potential cradles of extraterrestrial life.

Crucially, Sagan and his collaborators took a deliberately agnostic
approach to the detection of life, says astrobiologist Lisa Kaltenegger,
who heads the Carl Sagan Institute at Cornell University in Ithaca, New
York. “Of course he wants to find life, every scientist does,” she says.
“But he says, let’s take that wish and be even more cautious — because
we want to find it.” The existence of life was to be, in the words of
the paper, the “hypothesis of last resort” for explaining what Galileo
observed.

But even through this veil of scepticism, the spacecraft delivered.
High-resolution images of Australia and Antarctica obtained as Galileo
flew overhead did not yield signs of civilization. Still, Galileo
measured oxygen and methane in Earth’s atmosphere, the latter in ratios
that suggested a disequilibrium brought about by living organisms. It
spotted a steep cliff in the infrared spectrum of sunlight reflecting
off the planet, a distinctive ‘red edge’ that indicates the presence of
vegetation. And it picked up radio transmissions coming from the surface
that were moderated as if engineered. “A strong case can be made that
the signals are generated by an intelligent form of life on Earth,”
Sagan’s team wrote, rather cheekily.

A powerful control
Karl Ziemelis, now chief physical sciences editor at Nature, handled the
paper as a rookie editor. He says it remains one of his favourites — and
one of the hardest to get in. Editorial approval for the paper was far
from unanimous, because it was not obviously describing something new.
But, according to Ziemelis, that was mostly beside the point. “It was an
incredibly powerful control experiment for something that wasn’t really
on many people’s radar at the time,” he says.

“While the answer was known, it profoundly changed our way of thinking
about the answer,” says Kaltenegger. Only by stepping back and regarding
Earth as a planet like any other — perhaps harbouring life, perhaps not
— can researchers begin to get a true perspective on our place in the
Universe and the likelihood of life elsewhere, she says.

This false color image of the Eastern Coast of Australia was obtained by
the Galileo spacecraft, 1990.
No sign of civilization in Australia.Credit: NASA/JPL

It takes on a new importance given developments since the Galileo flyby.
In 1990, no planets orbiting stars other than the Sun were known. It was
another two years before astronomers conclusively reported the first
‘exoplanet’ orbiting a rotating dead star known as a pulsar4, and three
years more before they found5 the first around a Sun-like star, 51
Pegasi. Today, scientists know of more than 5,500 exoplanets, few of
which look like anything in the Solar System. They range from
‘super-Earths’ with bizarre geologies and ‘mini-Neptunes’ with gassy
atmospheres to ‘hot Jupiters’, huge planets whirling close to their
blazing stars.

When Sagan and his colleagues pointed Galileo at Earth, they invented a
scientific framework for looking for signs of life on these other worlds
— one that has permeated every search for such biosignatures since.
Kaltenegger still gives Sagan’s paper to her students to show them how
it is done. Life is the last, not first, inference to draw when seeing
something unusual on another planet, she tells them. Extraordinary
claims require extraordinary evidence.

The right mix for life
This lesson could not be more important today, as scientists stand on
the verge of potentially revolutionary, and perhaps monumentally
confusing, discoveries by the powerful James Webb Space Telescope
(JWST). The telescope is just beginning its remote exploration of the
atmospheres of dozens of exoplanets, hunting for the same sort of
chemical disequilibrium that Galileo spotted in Earth’s atmosphere. It
is already turning up early hints of biosignatures that might lead
scientists and the public astray.

For instance, JWST has sniffed out methane in the atmosphere of at least
one planet. That gas is a powerful signature of life on Earth, but it
can also come from volcanoes, no life required. Oxygen captures
scientists’ attention because much of it is generated by life on Earth,
but it can also be formed by light splitting apart molecules of water or
carbon dioxide. Finding the right combination of methane and oxygen
could indicate the presence of life on another planet — but that world
needs to be located in a temperate zone, not too hot nor too cold.
Getting the right mix of life-sustaining ingredients in a life-friendly
environment is challenging, Kaltenegger says.

The same is true for other intriguing mixes of atmospheric gases. Just
last month, astronomers sifting through JWST data reported finding
methane and carbon dioxide in the atmosphere of a large exoplanet called
K2-18 b. They suggested that the planet might have water oceans covering
its surface, and hinted at tantalizing detections of dimethyl sulfide, a
compound that, on Earth, comes from phytoplankton and other living
organisms6.

Headlines ran wild, with news stories reporting possible signs of life
on K2-18 b. Never mind that the presence of dimethyl sulfide was
reported with low confidence and needed further validation. Nor that no
water had actually been detected on the planet. And, even if water were
present, it might be in an ocean so deep as to choke off all geological
activity that could maintain a temperate atmosphere.

Building evidence
Challenges such as these led Jim Green, a former chief scientist at
NASA, to propose a framework in 2021 for how to report evidence for life
beyond Earth7. A progressive scale, from one to seven, for example,
could help to convey the level of evidence for life in a particular
discovery, he argues. Maybe you’ve got a signal that could result from
biological activity — that would just be a one on the scale. You’d need
to work through many more steps, such as ruling out contamination and
acquiring independent evidence of the strength of that signal before you
could get to level 7 and demonstrate a true discovery of life beyond Earth.

It could take a long time. A telescope might sniff out an intriguing
molecule, and scientists would argue about it. Another telescope might
be built to work out the context of the observation. Each brick of
evidence must be placed on top of another, each layer of mortar mixed
through the arguments, scepticism and agnosticism of many, many
scientists. And that’s assuming that life on another world resembles
that on Earth — an assumption underlying the conclusions drawn from
Galileo’s observations. “The uncertainty may last years or decades,”
Grinspoon says. Sagan, who died in 1996, would have loved it.

The same year that Galileo observed Earth, Sagan convinced NASA to point
another spacecraft in a direction the agency had not been planning. As
Voyager 1 raced past Neptune on its way out of the Solar System, it
turned its cameras back towards Earth and photographed a tiny speck,
gleaming in a sunbeam. This was the iconic Pale Blue Dot image that
inspired Sagan to ruminate in his 1994 book Pale Blue Dot: “That’s here.
That’s home. That’s us.”

That fragile gleaming pixel reshaped how humanity visualizes its place
in the Cosmos. So, too, did using Galileo to look for life on Earth,
says Kaltenegger: “This is how we can use our pale blue dot to provide a
template for the search for life on other planets.”

Nature 622, 451-452 (2023)

doi: https://doi.org/10.1038/d41586-023-03230-z

References